The sunlight reaching the earth includes infrared light, visible light, and ultraviolet light, of which 5 to 6% is ultraviolet light. The ultraviolet light has short wavelengths, which are thus high-energy electromagnetic waves. Therefore, the ultraviolet light is known to decompose many kinds of materials and to damage living organisms.
Therefore, ultraviolet shielding agents are used for protecting skin from inflammation or skin cancer due to exposure of the skin to harmful ultraviolet light. For this purpose, the ultraviolet shielding agents are added to cosmetics. Also, they are used to prevent pigments from fading due to decomposition by ultraviolet light. For this purpose, the ultraviolet shielding agents are mixed with paints; however, this may cause unnatural skin whitening and a color change of paints. This can be prevented by increasing the transparency of such cosmetics or paints in the visible light region. Therefore, the ultraviolet light is desirably blocked while the transparency in the visible light region is maintained.
The ultraviolet shielding agent comprising organic compounds as effective ingredients prevents the transmission of the ultraviolet light on account of the above organic compounds which absorb the ultraviolet light. For example, an ultraviolet absorbent composition comprising substituted N,N'-bis-aromatic formamidines is known (Japanese Patent Examined Publication No. 61-09993). However, the organic ultraviolet shielding agents have the problem that although they can absorb the ultraviolet light, they are at the same time likely to be decomposed by the ultraviolet light, with the result of an undesirable lowering of the shielding ability over time. Regarding their application to cosmetics, the kinds and amounts of the ultraviolet shielding agents are restricted owing to deleterious effects caused on human bodies, and thus it is difficult to achieve a good shielding performance within a controlled range.
On the other hand, the ultraviolet shielding agent comprising an inorganic compound contains inorganic fine particles and prevents the transmission of ultraviolet light by the absorbing ability and the scattering ability of the composition. The inorganic ultraviolet shielding agent is superior to the organic ultraviolet shielding agent because the composition containing the inorganic ultraviolet shielding agent is not decomposed by the ultraviolet light with the passage of time and has little effects on the human body.
However, since the inorganic ultraviolet shielding agents are present in the form of particles, it is more difficult with inorganic ultraviolet shielding agents when compared with organic ultraviolet shielding agents to block the ultraviolet light while maintaining high transparency in the visible light region.
In order to possess an effective light shielding ability in the ultraviolet light region while maintaining high transparency in the visible light region (light wavelengths of from 400 to 800 nm), the composition has to be microgranulated to give ultrafine particles capable of being highly dispersed so as to increase the ultraviolet scattering ability. However, in the case of using ultrafine particles, dispersion stability problems may arise due to the aggregation of the ultrafine particles and the catalytic activities of the ultrafine particles.
In order to improve dispersibility, the ultrafine particle surfaces may be coated with other materials For example, skin cosmetics comprising an oily cosmetic base material and a hydrophobic titanium oxide powder are known (Japanese Patent Examined Publication No. 59-15885). However, a suitable solvent has to be selected depending upon the properties of the materials coated on the surface. Also, since the particles are still ultrafine, the aggregation of the ultrafine particles can only be lowered to a limited extent even if the surface treatment is conducted. In publications other than those mentioned above, there have been known cosmetics containing a powder obtainable by coating titanium oxide with a particular amount of mixed hydrates comprising silicate hydrates and aluminum hydrates, titanium oxide being nearly spherical or irregularly shaped and having an average particle diameter of from 30 to 70 nm, wherein the surface of the hydrate-coated titanium oxide is optionally further coated with a silicone oil (Japanese Patent Laid-Open No. 2-247109). However, since in this publication the above powder is obtained by drying and pulverization of the product obtained after coating with the mixed hydrates comprising silicate hydrates and aluminum hydrates or after coating the surface of the powder with the silicone oil, it is extremely difficult to pulverize the titanium oxide ultrafine particles to the size of the primary particles, because the titanium oxide ultrafine particles are aggregated showing a large particle diameter, so that the transparency and the ultraviolet shielding ability of the above obtained powder are lowered. Also, since fresh, uncoated surfaces appear after the pulverization process is carried out, the water-repellent ability or the oil-repellent ability of these surfaces is undesirably lowered. Such technological problems arise in maintaining the dispersibility of the ultrafine particles stable. Therefore, it is increasingly important to find a way to achieve a high dispersibility of the ultrafine particles and maintain it at that level.
Also, for the purposes of providing cosmetics comprising ultrafine particles wherein the ultrafine particles are easily and uniformly dispersed and the problem of the difficult handling of the ultrafine particles powder is eliminated, starting materials for cosmetics comprising metal oxide ultrafine particles having a particle diameter of not more than 0.1 .mu.m, a dispersion medium, and a dispersant, wherein the content of the ultrafine particles is not less than 10% by weight, have been known (Japanese Patent Laid-Open No. 6-239728) However, although the problems regarding the aggregation of the ultrafine particles and the deterioration of the dispersant, the dispersion medium, and cosmetics base materials caused by the catalytic activity of the metal oxide ultrafine particles have been known, they have neither been addressed nor solved in this publication. Moreover, the content of the metal oxide ultrafine particles in the starting materials for cosmetics is limited to not less than 10% by weight in order to control the amounts in the overall cosmetics in this publication. However, as long as the metal oxide ultrafine particles are uniformly and stably dispersed, the function of the metal oxide ultrafine particles is high, and the content of the metal oxide ultrafine particles needs not be limited to a minimum amount of 10% by weight in the starting materials for any kind of cosmetics.
Therefore, in order not to lower the ultraviolet scattering ability by the aggregation of the inorganic ultrafine particles, composites of the inorganic ultrafine particles are often formed with other relatively large carrier particles. For example, a thin flaky material dispersed with metal compound fine particles is known (Japanese Patent Laid-Open No. 63-126818). However, this publication does not disclose a specific construction of the fine particles for improving both the shielding ability in the ultraviolet light region and the transparency in the visible light region
Furthermore, composite fine particles comprising ultrafine particles dispersed in and supported by the solid material are proposed. Conventional ultraviolet shielding composite fine particles include, for example, a composite powder in which a fine particle powder, such as TiO.sub.2, is uniformly dispersed in plate particles of metal oxides, such as SiO.sub.2 (Japanese Patent Laid-Open No. 1-143821); and composite particles in which a zirconium oxide powder or an aluminum oxide powder is carried on a surface of the matrix particles comprising such materials as nylon resins, silicone resins, and silicon oxide, and a titanium oxide powder or a zinc oxide powder dispersed in an inner portion of the matrix particles (Japanese Patent Laid-Open No. 2-49717).
However, in order to use the above composite particles as ultraviolet shielding agents, the composite particles are usually dispersed in a medium in the actual environment. In this case, since the metal oxides, such as titanium oxide, contained in the composite particles have catalytic activity, the deterioration of the medium is likely to take place. Also, when the difference between the refractive index of the composite particles and that of the medium is large, light scattering takes place at an interface of the composite particles and the medium, thereby making both the transparency in the visible light region and the shielding ability in the ultraviolet light region poor. Although these problems need to be solved, they have not been considered in the above publications.
In order to suppress the catalytic activities of the ultrafine particles, methods for coating a surface of the ultrafine particles with various materials have been used. For example, Japanese Patent Laid-Open No. 5-70331 discloses the preparation of cosmetics comprising fine particle powder wherein a basic compound and at least one of a hydrocarbon compound having a boiling point of from 100 to 200.degree. C. and a silicone having a particular molecular structure are added during the production of titanium hydroxide which is obtainable by hydrolysis of a titanium alkoxide. However, in order to produce titanium hydroxide fine particle powder, the production process must comprise drying and pulverization processes, which results in a large particle diameter of the obtained titanium hydroxide fine particles. Thus, it is difficult for the particles to be highly effective in scattering ultraviolet light B (light wavelengths of from 280 to 320 nm), while maintaining high transparency of in the visible light region.
Also, the above publication has neither considered nor disclosed any method for producing ultrafine particles for the titanium hydroxide particles or methods for dispersing the ultrafine titanium hydroxide particles in cosmetics, in order to satisfy both high transparency in the visible light region and high shielding ability to the ultraviolet light. Moreover, in the ultraviolet shielding materials disclosed in this publication, titanium hydroxide or titanium oxide presumably absorbs the ultraviolet light B (light wavelengths of from 280 to 320 nm). This ultraviolet light B only penetrates the epidermis and a relatively upper layer of the dermis, causing sunburn or skin cancers. However, titanium hydroxide or titanium oxide does not at all absorb, light having wavelengths of from 350 to 400 nm, which are closer to those of the visible light of the ultraviolet light A (light wavelengths of from 320 to 400 nm). The ultraviolet light A reaches skin layers beyond the dermis, and produces suntan or fibrous modifications in the dermis. In other words, the ultraviolet absorbents disclosed in this publication mainly exhibit absorption of the ultraviolet light B by titanium hydroxide or titanium oxide, but their ultraviolet absorption effects are limited to a light wavelength of up to about 300 nm for an anatase-type titanium oxide and to a light wavelength of up to about 320 nm for a rutile-type titanium oxide.
Of the ultraviolet light reaching the earth, the energy proportion of the ultraviolet light A is about 15 times that of the ultraviolet light B. Therefore, in view of the above energy proportion of the ultraviolet lights A and B, it is important to shield both the ultraviolet light A as well as the ultraviolet light B, and simply shielding the ultraviolet light B is not sufficient. Moreover, it is becoming increasingly important to shield both the ultraviolet light B and the ultraviolet light A, while maintaining a high transparency in the visible light region. In particular, in the case where the ultraviolet light A is shielded, it is important to shield light wavelengths of from 350 to 400 nm, which are closer to light wavelengths of the visible light.
As mentioned above, in order to achieve an effective shielding ability in the ultraviolet light region which maintaining and high transparency in the visible light region, the ultraviolet shielding materials are made ultrafine so that they are present in a highly dispersed state. In order to further improve the transparency in the visible light region, it is important for the difference between the refractive indices of the ultraviolet shielding materials and those of surrounding media to be kept small. As the refractive index of the titania-containing composite powders disclosed in Japanese Patent Laid-Open Nos. 1-143821 and 6-116119 is limited to certain ranges determined by the compositional ratios of the components, the number of suitable dispersion media whose refractive index corresponds to the refractive index of the composite powder is inevitably limited as well. Therefore, great problems have been encountered in controlling the refractive index of the composite powder so as to match it with the refractive index of the dispersion medium used. Therefore, an effective means for solving these problems is in great demand.
Further, Japanese Patent Laid-Open No. 4-65312 is concerned with metal compound-containing porous silica bead, the production method thereof, and the powder deodorant produced. In this publication, the fine particles of the metal compounds having a primary particle diameter of from 0.001 to 0.3 .mu.m are contained in the porous silica bead in an amount of from 0.1 to 30% by weight, and the porous silica bead contain substantially no voids of not less than 0.3 .mu.m. In this case, when the fine particles of the metal compounds contained therein are suitably selected so as to have a refractive index close to the refractive index of silica (the refractive index being in the range of from 1.4 to 2.0), silica particles with further improved transparency can be obtained. However, the publication only discloses the range for the refractive index of the metal compound fine particles contained in the inner portion of the composite particles, but none of the total refractive index of the composite particles.
As explained above, in order to solve the problems inherent in the ultraviolet shielding agents comprising the ultrafine particles, several attempts have been made to use composites mainly comprising metal oxides. However, many of the compounds exhibiting good ultraviolet absorption properties, such as TiO.sub.2 and ZnO, have relatively high refractive indices, so that the composite fine particles incorporating these ultrafine particles have refractive indices notably higher than aqueous solutions, conventional organic solvents, polymers, etc. When the above composite fine particles are dispersed in a medium, the present inventors have found that light scattering in the visible light region takes place at the interface of the composite fine particles and the medium, whereby the transparency of the medium is drastically lowered. However, a technical method of controlling the refractive index of the ultraviolet shielding particle of the composite fine particles has not been proposed so far.
In the fields of resin fillers, fluorine-based inorganic compounds, such as MgF.sub.2 and CaF.sub.2, or fluorine-based organic polymeric compounds, such as polyethylene tetrafluoride, which are known as low-refractive index materials having high transparency, are added to powders, etc. as starting materials to lower their refractive indices.
For instance, Japanese Patent Laid-Open No. 4-85346 discloses a glass powder, used as a transparent inorganic powder for resin fillers, comprising metal oxides, such as SiO.sub.2, Al.sub.2 O.sub.3, B.sub.2 O.sub.3, BaO, SrO, ZnO, and MgO, and metal fluorides, the glass powder having a refractive index (n.sub.D) adjusted in the range of from 1.44 to 1.70. The publication discloses that since the glass powder has a high light transmittance and does not show strong alkalinity, the resins do not undergo any substantial modification, and are significantly stabilized in resin hardening. However, the publication merely discloses that a highly transparent inorganic powder for resin fillers is obtainable by changing the compositional ratio of the materials, and the above metal oxides, etc. are not present as particles in the final product powder owing to the high-temperature melting production, and this publication does not refer to the ultraviolet shielding ability. Further, this publication does not disclose that the composite fine particles comprise aggregates of two or more kinds of fine particles as in the present invention or that the composite fine particles have the compositional dependency with respect to an average refractive index of the composite fine particles.